1. Field of the Invention
The present invention relates generally to an anti-reentry apparatus and method that can favorably influence the beating of ineffective hearts, especially hearts with pathological conditions that interfere with normal rhythmicity, electrical conduction, and/or contractility by causing ventricular fibrillation. The present invention particularly relates to an anti-reentry apparatus and method that provides simultaneous or progressive biphasic stimulation at multiple sites in one or both ventricles.
2. Background Information
Heart disease and malfunction is a major killer of men and women in America. A variety of pathologies can affect the beating patterns of a heart, and thereby predispose it to developing ventricular fibrillation. Prior to the occurrence of such a severe and ineffective rhythm, conventional pacemakers can be used to treat, for example, such disorders as sino-atrial (SA) node block, A-V block, and multiple independent sites of contraction in the ventricles (also termed ectopic foci), which, in the extreme, can lead to life threatening ventricular fibrillation. Conventional pacemakers often will control and prevent the recurrence of ectopic foci by preprogrammed stimulation of (usually) the right ventricle via a single electrode. Some pacemakers also employ a second electrode that is dedicated to the left ventricle. In addition, conventional pacemakers utilize a range of circuit logic patterns to counter specific problems that are encountered in the more common pathologies.
However, conventional single ventricular electrode technologies, including the use of a separate single electrode to each ventricle, fail in cases in which ventricular fibrillation has ensued (particularly when the fibrillation is due to multiple random reentry), and single site stimulation does not entrain sufficiently large areas of surrounding tissue to produce the concerted contraction that is necessary for optimal efficiency in pumping blood. In such cases of ventricular fibrillation from multiple random reentry, the patient is put in grave jeopardy for the basic reason that virtually all of the body functions depend on delivery of blood to the tissues in order to supply oxygen and nutrients, and also to carry away metabolic waste products. Failure to correct such a condition, where the rhythm is so far from optimal, results in the patient being in substantial danger of dying in a very short period of time. Though cardioversion/defibrillation may be employed, including that preprogrammed in the control logic for automatic activation in some pacemaker-defibrillators, such protocols typically require large doses of electrical energy to the patient. In addition to producing extreme discomfort and sharp pain, these large doses of electrical energy often also produce cardiac damage. The voltage for standard internal defibrillation/cardioversion is from 150 to 800 volts, corresponding to approximately 10-35 joules.
Several approaches to these problems have been disclosed. One approach is to stimulate greater portions of ventricular myocardium by utilizing larger electrodes so that greater portions of myocardium are simultaneously stimulated. For example, U.S. Pat. No. 5,411,547 to Causey, III discloses the use of defibrillation electrode patches for more efficient bipolar cardiac stimulation. In addition, the use of large, plate-like electrodes for defibrillation and cardioversion is well known. However, the use of such larger electrodes suffers from the problem of delivery of large doses of electrical energy that produce great discomfort to the patient and the possibility of tissue damage.
Yet another approach is to use multiple individual electrodes appropriately placed about the ventricles. For further details, refer to the following U.S. Pat. Nos. 5,649,966 to Noren et al., 5,391,185 to Kroll, 5,224,475 to Berg et al., 5,181,511 to Nickolls et al., and 5,111,811 to Smits. Though these patents disclose the use of multiple electrodes, they do not disclose or suggest their use for gradually (yet quickly) entraining the various reentrant foci that can exist in pathological ventricles by stimulating in a progressive pattern that mimics the normal wave of depolarization that occurs in the heart.
Thus, a need exists for an anti-reentry apparatus and method that will require the use of less electrical current/voltage than is typically used for defibrillation and cardioversion in order to decrease the likelihood, or at least the severity, of tissue damage. A need also exists for an anti-reentry apparatus and method that will simultaneously stimulate greater portions of ventricular myocardium to increase the probability of ventricular conversion (particularly in the presence of multiple random reentry), but with delivery of lower doses of electrical energy per stimulation, which, consequently, will prolong the life of the apparatus""s batteries and decrease myocardial soft tissue damage. A need also exists for such an anti-reentry apparatus and method that not only will produce the vitally needed improvement in cardiac pumping efficiency, but additionally will simultaneously lower the probability of tissue damage, and provide greater comfort for the patient. In addition, a need exists for an anti-reentry apparatus and method that progressively stimulates the ventricles in a manner that mimics the normal cardiac wave of depolarization, thereby providing rapid control and reversion of cardiac rhythm to a normal beating pattern.
In view of the foregoing limitations in the art, it therefore is an object of the present invention to provide an apparatus and method that more efficiently and quickly entrains larger areas of myocardium to promote ventricular conversion, particularly in patients suffering from episodes of multiple random ventricular reentrant foci that produce, or may produce, ventricular fibrillation.
It is another object of the present invention to provide an apparatus and method that, while entraining larger areas of myocardium, does so with smaller doses of electrical energy than typically are used in defibrillation and cardioversion.
It is yet another object of the present invention to provide an apparatus and method that, while entraining larger areas of myocardium, does so by stimulating in a progressive pattern that mimics the normal wave of depolarization of the heart.
It is a further object of the present invention to provide an apparatus and method that, while entraining larger areas of myocardium, does so with less stress on the heart and greater comfort to the patient.
It is yet another object of the present invention to provide an apparatus and method that, while entraining larger areas of myocardium, does so with less damage to cardiac tissue.
It is yet a further object of the present invention to provide an apparatus and method that, while entraining larger areas of myocardium, also promotes greater myocardial blood pumping efficiency.
It is yet a further object of the present invention to provide an apparatus and method that entrains larger areas of myocardium by using multiple electrodes that provide biphasic stimulation.
Pacemakers, which utilize low energy stimulation pulses, constitute a separate and distinct art from cardioverters/defibrillators, which utilize stimulation pulses of much larger energyxe2x80x94even when the electrodes are positioned directly on the heart. Thus, according to conventional practice, more energy is required to entrain the entire heart (cardioversion/defibrillation) than to exogenously employ the traditional pacemaker that typically utilizes the natural cardiac conducting fibers and/or endogenous pacemaker(s) to control the beating of a heart that is only slightly xe2x80x9cout of synchxe2x80x9d relative to the more dangerous rhythmicity disorders that often result in extensive fibrillation.
An intermediate ground is demonstrated by the present invention. By using multiple electrodes and applying biphasic stimulation, one or both ventricles may gradually (yet quickly) be entrained to beat more normally in the face of multiple random reentry, even though the stimulation energy level used is lower than that generally used for cardioversion/defibrillation.
Thus, the present invention accomplishes the above objectives by utilizing multiple electrodes that contact multiple ventricular areas 1) for simultaneous biphasic stimulation, or 2) for progressive biphasic stimulation, that is, the mimicking of the physiological patterns of electrical current flows or waves of depolarization in the myocardium. The control circuit logic can activate the multiple site, biphasic ventricular stimulation upon the occurrence of A-V block in a patient known to be susceptible to multiple random ventricular reentrant foci, or upon the direct or indirect sensing of ventricular fibrillation. For example, direct sensing of ventricular fibrillation can be based on data from multiple ventricular sensing electrodes, and indirect sensing can be based on any of various functional parameters, such as arterial blood pressure, size and/or presence of an R wave, rate of the electrogram deflections, or the probability density function (PDF) of the electrogram.
The present invention accomplishes the above objectives through the use of multiple site, biphasic ventricular stimulation in one or both ventricles to 1) gradually (yet quickly) entrain and interrupt substantially all of the multiple random reentrant circuits that are present; or, failing that, 2) reduce the number of such reentrant circuits to a level at which much smaller stimuli may be used than in conventional defibrillation/cardioversion to convert the rhythms to more normal ones, and thereby produce coordinated and efficient cardiac function.
The first and second phases of stimulation consist of an anodal pulse (first phase) followed by a cathodal pulse (second phase). In a preferred embodiment, the first phase of stimulation is an anodal pulse at maximum subthreshold amplitude and for a long duration in order to precondition the myocardium for subsequent stimulation, and the second phase of stimulation is a cathodal pulse with a short duration and a high amplitude. Additional embodiments of the first phase include, but are not limited to, the use of ramped pulses, a series of short duration square wave pulses, anodal pulses that are less than the maximum subthreshold amplitude, and pulses whose magnitudes decay from an initial subthreshold amplitude to a lower amplitude, where the shape of the decay can be linear or curvilinear. It is to be understood that the use of the phrase xe2x80x9cmedium energyxe2x80x9d stimulation or pulse refers to electrical stimulation or electrical pulses in which the magnitude of the voltage of the electrical stimulation/pulse is lower in magnitude than that used in typical defibrillation/cardioversion.